Recent work has shown the benefits of panoramic imaging, which is able to capture a large azimuth view with a significant elevation angle. If instead of providing a small conic section of a view, a camera could capture an entire half-sphere or more at once, several advantages could be realized. Specifically, if the entire environment is visible at the same time, it is not necessary to move the camera to fixate on an object of interest or to perform exploratory camera movements. Additionally, this means that it is not necessary to stitch multiple, individual images together to form a panoramic image. This also means that the same panoramic image or panoramic video can be supplied to multiple viewers, and each viewer can view a different portion of the image or video, independent from the other viewers.
One method for capturing a large field of view in a single image is to use an ultra-wide angle lens. A drawback to this is the fact that a typical 180-degree lens can cause substantial amounts of optical distortion in the resulting image.
A video or still camera placed below a convex reflective surface can provide a large field of view provided an appropriate mirror shape is used. Such a configuration is suited to miniaturization and can be produced relatively inexpensively. Spherical mirrors have been used in such panoramic imaging systems. Spherical mirrors have constant curvatures and are easy to manufacture, but do not provide optimal imaging or resolution.
Hyperboloidal mirrors have been proposed for use in panoramic imaging systems. The rays of light which are reflected off of the hyperboloidal surface, no matter where the point of origin, all converge at a single point, enabling perspective viewing. A major drawback to this system lies in the fact that the rays of light that make up the reflected image converge at the focal point of the reflector. As a result, positioning of the sensor relative to the reflecting surface is critical, and even a slight disturbance of the mirror will impair the quality of the image. Another disadvantage is that the use of a perspective-projections model inherently requires that, as the distance between the sensor and the mirror increases, the cross-section of the mirror must increase. Therefore, in order to keep the mirror at a reasonable size, the mirror must be placed close to the sensor. This causes complications to arise with respect to the design of the image sensor optics.
Another proposed panoramic imaging system uses a parabolic mirror and an orthographic lens for producing perspective images. A disadvantage of this system is that many of the light rays are not orthographically reflected by the parabolic mirror. Therefore, the system requires an orthographic lens to be used with the parabolic mirror.
The use of equi-angular mirrors has been proposed for panoramic imaging systems. Equi-angular mirrors are designed so that each pixel spans an equal angle irrespective of its distance from the center of the image. An equi-angular mirror such as this can provide a resolution superior to the systems discussed above. However, when this system is combined with a camera lens, the combination of the lens and the equi-angular mirror is no longer a projective device, and each pixel does not span exactly the same angle. Therefore, the resolution of the equi-angular mirror is reduced when the mirror is combined with a camera lens.
Ollis, Herman, and Singh, “Analysis and Design of Panoramic Stereo Vision Using Equi-Angular Pixel Cameras”, CMU-RI-TR-99-04, Technical Report, Robotics Institute, Carnegie Mellon University, January 1999, disclose an improved equi-angular mirror that is specifically shaped to account for the perspective effect a camera lens adds when it is combined with such a mirror. This improved equi-angular mirror mounted in front of a camera lens provides a simple system for producing panoramic images that have a very high resolution. However, this system does not take into account the fact that there may be certain areas of the resulting panoramic image that a viewer may have no desire to see. Therefore, some of the superior image resolution resources of the mirror are wasted on non-usable portions of the image.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.